Twenty years ago, a new kind of prison was taking America by storm. The supermax prison was designed to incapacitate dangerous criminals by locking them down in stark isolation, sometimes for years on end.

So have the supermaxes lived up to their promise of stopping violent criminals?

Pelican Bay State Prison in California is one of America’s biggest supermaxes, but it is not just a supermax. It’s also headquarters to some of America’s biggest and most violent prison gangs. Experts say these gangs control crime far outside prison walls.

The idea behind the supermax is not new. Nearly 200 years ago, America’s first penitentiary in Philadelphia locked men in solitary confinement, giving them only a Bible. Since then, America has used prisons to either punish criminals, or reform them.